Globally, air conditioning and electric fans contribute to nearly 20% of building electricity consumption, representing roughly 10% of the world's total electrical use, reports the International Energy Agency in The Future of Cooling.
Scientists at King Abdullah University of Science and Technology have developed and tested an alternative cooling technology called NESCOD that operates without traditional refrigeration. This system leverages ammonium nitrate dissolved in water to generate a cooling effect, with sunlight then used to crystallize and recycle the salt.
Experimental results showed that the ammonium nitrate solution's temperature dropped to about −2.4°C within 20 minutes. Additionally, calculations indicated cooling capacities reaching up to 191 watts per square meter during solar-powered regeneration processes.
Endothermic Reaction of Ammonium Nitrate Enables Cooling
The cooling mechanism initiates as ammonium nitrate dissolves in water. This endothermic dissolution absorbs heat from the surroundings, causing the solution’s temperature to decrease.
Before selecting ammonium nitrate for their system, the team compared its performance against various salts. Their research published in Energy & Environmental Science evaluated potassium nitrate, sodium nitrate, ammonium chloride, potassium chloride, and potassium bromide.
The evaluation considered each salt’s solubility in water and the heat absorption during saturation. Among these, ammonium nitrate stood out with the greatest calculated cooling capability.
Its notable solubility allows a substantial amount of salt to dissolve in a small water volume. The study reports an enthalpy of saturated solution of 187.6 kilojoules per kilogram for ammonium nitrate.
Initial testing assessed how rapidly temperature drops could be achieved. Using a Dewar flask, the ammonium nitrate solution reached approximately −2.4°C after 20 minutes, while the flask’s air temperature declined more gradually, hitting 9.1°C within 51 minutes.
Further experiments involved a metal cup inside an insulated polystyrene case, surrounded by ammonium nitrate, with water added incrementally to maintain ongoing dissolution rather than a sudden cooling burst.
The cup’s temperature dropped to about 6.2°C after 26 minutes and further decreased to around 3.6°C after 220 minutes. It held temperatures below 10°C for over eight hours and stayed beneath 15°C for upwards of 15 hours.
Solar Energy Drives Salt Recovery Process
Following the cooling phase, ammonium nitrate remains dissolved. To reuse it, the system must extract water and recrystallize the salt.
Researchers engineered a three-dimensional solar regenerator that captures sunlight to heat the solution and promote evaporation. As water vaporizes, the solution concentrates, and crystals of ammonium nitrate form and can be harvested.
This regenerator distinguishes zones for sunlight absorption and crystal formation. Its base serves as a light-absorbing platform, while the exterior area facilitates evaporation and crystallization.
Initial tests encountered salt buildup issues; ammonium nitrate created a dense crust that limited fresh solution circulation, reducing evaporation efficiency.
The team addressed this by introducing sodium 4-vinylbenzenesulfonate (SVBS) to modify crystal nucleation and adding a PTFE film to prevent salt accumulation on the sunlight entry points.
These improvements yielded an evaporation rate of nearly 2.2 kilograms of water per square meter per hour and a salt recovery rate close to 4.6 kilograms per square meter per hour under standard sunlight intensity.
Using these values, the researchers calculated cooling output to be as high as 191 W/m², assuming an ambient temperature of 35°C and a final solution temperature of 25°C.
They also captured condensed water vapor during regeneration; impurity checks showed ion and organic carbon levels below 1 part per million.
Decoupled Cooling and Solar Regeneration Offer Flexibility
NESCOD’s design separates cooling from the regeneration stage. Sunlight-powered crystallization can be conducted independently, storing solid ammonium nitrate for later use. The salt then reinitiates cooling upon mixing with water.
This separation allows deployment in different locations. For example, solar regeneration units can occupy rooftops while cooled salt is used inside buildings.
The researchers tested both localized cooling and insulated storage scenarios. One highlighted application involved chilling a mattress rather than cooling an entire room.
The system operates without electrical compressors. It is fully passive, with no need for high-pressure compression or moving mechanical parts during cooling.
The IEA’s report highlights the urgency: by 2050, about two-thirds of global households may have air conditioners, with China, India, and Indonesia comprising half the demand. Without improved efficiency, space cooling electricity use could more than triple.
In their tests, the insulated container stayed below 10°C for more than 8 hours and under 15°C for over 15 hours. Solar regeneration yielded salt recovery at roughly 4.6 kilograms per square meter per hour under full sunlight.
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